1. Field of the Invention
This invention relates to implantable hearing assistance systems for hearing impaired persons, and in particular, to a method of creating a coupling between an implantable component and a structure of the ear.
2. Description of Related Art
In a patient with normally functioning anatomical hearing structures, sound waves are directed into an ear canal by the outer ear and into contact with the tympanic membrane. The tympanic membrane is located at the terminus of the ear canal. The pressure of the sound waves (acoustic sound energy) vibrates the tympanic membrane resulting in the conversion to mechanical energy. This mechanical energy is communicated through the middle ear to the inner ear by a series of bones located in the middle ear region. These bones of the middle ear are generally referred to as the ossicular chain, which includes three primary structures, the malleus, the incus and the stapes. These three bones must be in functional contact in order for the mechanical energy derived from the vibration of the tympanic membrane to be transferred through the middle ear to the inner ear. If these three bones do not effectively communicate the mechanical energy through the middle ear, the patient suffers from a conductive hearing loss.
Various implantable devices have been developed to assist the hearing impaired patient. Some implantable hearing assistance systems use an acoustic microphone located in or near the ear to convert acoustic sound energy into an electrical signal. The electric signal is amplified, modulated, and then directly communicated by an output transducer to the inner ear to stimulate the cochlea to assist in hearing. Alternatively, the amplified signal is communicated to a transducer for conversion to mechanical energy for vibratory application to the stapes or cochlea. The microphone may be located externally, subdermally adjacent the ear, or within the external auditory canal. The output transducer is commonly connected to the ossicular chain. Vibrations are emitted from the output transducer into and through the ossicular chain to the cochlea.
Other implantable devices include partial middle ear implantable or total middle ear implantable devices, cochlear implants, and other hearing assistance systems that use components disposed in the middle ear or inner ear regions. These components may include an input transducer for-receiving sound vibrations or an output transducer for providing mechanical or electrical output stimuli based on the received sound vibrations. Piezoelectric transducers are one example of a class of electromechanical transducers that require contact to sense or provide mechanical vibrations. For example, the piezoelectric input transducer in U.S. Pat. No. 4,729,366, issued to D. W. Schaefer on Mar. 8, 1988, contacts the malleus for detecting mechanical vibrations. In another example the piezoelectric output transducer in the '366 patent contacts the stapes bone or the oval or round window of the cochlea.
Devices for assisting the hearing impaired patient range from miniaturized electronic hearing devices that may be adapted for placement entirely within the auditory canal, or implantable devices which may be completely or partially implanted within the skull. For those hearing systems, or portions of hearing systems, designed for complete subcranial implantation, a challenge has existed to adapt the implantable device for optimal mounting to the unique patient morphologies (including both naturally occurring as well as those created by surgical processes) among patients. Known implantable devices having elements that perform a support or mounting function are typically rigidly mounted to a bone within the middle ear region. Difficulties have arisen with the use of implantable devices in facilitating the fine adjustments necessary to properly position and configure the support assembly and attached transducers so as to contact an auditory element and thus vibrate a portion of the ossicular chain. Such devices present a particular problem in that positioning, or docking, of the transducer against the auditory element in this stable configuration requires extremely fine adjustments that are difficult given the location of the auditory elements and the attendant's lack of maneuvering room.
A middle ear implantable hearing assistance system typically includes, at least, an input device, such as a sensor transducer, an output device, such as a driver transducer, an electrical connection between the devices and a coupling of at least one of the devices to an element of the middle ear. Typically, the coupling between a transducer and the middle ear element is mechanical. The transducer communicates with the middle ear element via the mechanical coupling and the mechanical coupling is, therefore, critical to the efficacy of the hearing aid system. Proper positioning of the transducer and good contact between the transducer and ossicle is essential to properly transducing the received mechanical energy into a resulting electrical signal for hearing assistance processing.
There is a need in the art to ascertain whether too much force between the transducer and the ossicle, for example the malleus, may mechanically load the vibrating ossicle and attenuate the desired mechanical vibration signal or alter its frequency characteristics. It may be that, in an extreme case, too much force may damage or break either the ossicle or the transducer. It is also possible that too little force between the transducer and the ossicle may be insufficient to detect the mechanical vibration signal, and result in a complete loss of signal detection if the transducer and the ossicle become dissociated.
It is desirable for a device to accommodate the morphology of the ossicle or tissue which it is connecting (directly or indirectly) as opposed to devices of the prior art that do not take into account the morphological differences of each patient. Such prior art devices either harm the patient by not taking into account, fully, the detrimental impact on tissue patency caused by its structural method of attachment, are nonfunctional, or lose functioning ability with drops of pressure. Specifically, when a transducer is too loosely coupled to the ossicle, there is no signal and, conversely, when a transducer is too tightly coupled to the ossicle, there may be a less than optimum frequency response or harm to the tissue.
Prior art coupling mechanisms used, for example, in coupling a transducer to an ossicle, have a variety of problems. Typically, biasing, crimping, or adhesives have been used to attach to an ossicle. Biasing may result in a connection which is too loose because of the difficulty in determining the extent of the biasing. Over a patient's lifespan, muscles, tissue, and ligaments may stretch and cause the biasing to become loose. Additionally, even if the biased element is not loose during everyday activity, it may become loose and lose contact altogether with a change in pressure, such as in an elevator or an airplane. Crimping has similar problems. It is difficult to determine when the element has been adequately crimped to the ossicle. If the element is too tightly crimped to the ossicle, the blood vessels lose patency and bone rotting may occur. If the element is too loosely crimped to the ossicle, there may be resonances and a poor frequency response.
Adhesives, as well, have evidenced problems in coupling a transducer to an ossicle. One problem associated with adhesives is that, although affecting good fixation to the ossicle without damaging the ossicle, the hard fix of the transducer to the ossicle can inhibit natural movement of the ossicle. The ossicular elements of the middle ear have a complex range of motion. Specifically, each ossicle has yaw, pitch, and roll movement. When a device is coupled to the ossicle with hard fixation, at least one range of movement tends to be limited. This can attenuate, for example, the vibrations sensed by an input transducer and, therefore, decrease the efficacy of the implantable hearing assistance device.
Similar problems occur when coupling an ossicle to a passive prosthesis. A passive prosthesis is used when one or more of the malleus, incus, or stapes is partially or completely removed or damaged. The passive prosthesis maintains functional contact to transfer the mechanical energy derived from the vibration of the tympanic membrane through the middle ear to the inner ear.